If you mean add more cels or loops, simply go to insert or append cel under the cel menu. Same for loops.
If you mean choose which cel or loop to be used in your logic, use this:
set.cel(oa,1); for example. This says for object a, display cel number 1.
the same can be used for loops:
set.loop(oa,1);
The help file in agi studio is quite useful as well.